Mostly comes down to marketing. There's a limited amount of time to put on races for all categories, so promoters allocate that time in a fashion that gets them the best turnout.
One factor to consider though is that an open Masters Race is pretty much a Pro 1,2,3 race. So most of the people in the field will be used to doing longer races than a 4/5 field.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.